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A D-Brane is a strongly coupled -dimensional object that appears in String Theory, whose existence is required by T-Duality and is motivated by Ramond-Ramond Charges.

From Ramond-Ramond Charges


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The worldsheet of a string can couple to a Neveu-Schwarz B-field: Now, the is the EM-charge.

The worldsheet of a string can couple to graviton field (spacetime metric):

You can change the "" to any way you like, in terms of the tension/Regge Slope parameter/string length etc.

For a dilaton field,

Forget the conformal invariance for the time being. But what about Ramond-Ramond potentials? All is fine with the Ramond-Ramond Fields, but the Ramond-Ramond Potentials are associated with the Ramond-Ramond field and it is intuitive (and quite clear) that they can't couple similarly to the worldsheet. But it can for a worldhhypervolume, as long as the world-hypervolume is not 2-dimensional. It would then be given by:

Note the similarity to the other couplings. Of course, this does not really neccessitate the existence of D-Branes though.

From T-Duality

Considering T-Duality on open strings immediately realises that open strings with Newmann boundary conditions ("Free" strings) are mapped to those with Dirchilet boundary conditions ("Bound" strings). These "bound" strings must be attached to D-branes. Therefore, strings with Newmann boundary conditions in a String Theory would have Dirchilet boundary conditions in the T-dual theory.

Therefore, this requires the existence of D-Branes.